In relation to some diseases patients must inject a medicament on a regular basis such as once weekly, once daily or even a plurality of times each day. In order to help patients overcome fear of needles, fully automatic injection devices have been developed with the aim oft making the use of the injection device as simple as possible. Such devices are typically designed such that a user shall position the injection device onto the injection site and activate the device. Such activation causes the device to insert a needle into the skin, eject a dose of the medicament and subsequently move the needle into a shielded position.
Generally, for injection devices of the above type, main attention has been directed towards devices equipped with a glass cartridge where a needle cannula is fixedly attached to the outlet end of a cartridge. Such needle cannula is initially being covered in a sterile way by a cap member that during storage acts as a stopper for the needle cannula, and which requires removal prior to use. Typically, these devices further include a needle shield portion for shielding the needle before and/or after use. Disclosure of such devices is included in U.S. Pat. Nos. 7,449,012, 7,717,877 and WO2008/116688.
Some manufacturers prefer the type of cartridge having a pierceable septum which during storage provides a seal for the cartridge outlet and where the septum, upon use, is pierced by a needle cannula. Prior art devices using this type of cartridge are disclosed in U.S. Pat. Nos. 2,752,918, 5,658,259, 6,743,203, 6,210,369 and WO94/07553. Devices of that type hold a needle assembly and a cartridge in a separated storage configuration which upon activation of the device allows for subsequent connection to establish fluid communication between cartridge and needle assembly. In addition, automatic penetration of the needle into the skin of the user for subsequent automatic delivery of the medicament is typically incorporated.
While the above devices aim at providing a high level of automation, injection devices that provide automatic insertion of the needle into the dermis also prevent the user from controlling the insertion, which can lead to uneasiness for the user.
Injection devices that provide automatic delivery of the medicament, i.e. auto-injectors, typically use a drive spring as driving force for the injection. Before use, the drive spring will be held in a pre-tensioned position from which it is released upon activation of the device. After activation the drive spring uses the energy from the tension to drive forward the piston of a cartridge.
One problem associated with auto-injectors having needle shield operated triggering is that the release mechanism typically relies on at least one component that is exerted to excessive forces and that maintains the drivespring in a state where the plunger can be released for expelling the medicament of the cartridge. The triggering principle typically relies on at least one component that is deformed to unlock for releasing energy from the drive spring. Due to the excessive forces provided by the drive spring such principle often results in non-optimal performance of the needle shield movement.
Having regard to the above-identified prior art devices, it is an object of the present invention to provide an autoinjector that is improved regarding needle shield triggering by movement of a needle shield and which enables improved control of the device during operation.
Yet additional further objects of the invention are to provide measures for obtaining devices having a superior performance and, at the same time, enabling manufacture at a reduced cost.